1. Field of the Invention
The present invention relates to a method of and an apparatus for correcting the gradation of an image represented by image data.
2. Description of the Prior Art
In a conventional image processing apparatus such as a process scanner, gradation correction is performed on image obtained by reading an original with a photoelectric converter thereby to obtain a recorded image having desired gradation expression. In order to perform such gradation correction, it is preferable not to use a fixed gradation correction table, but to employ a gradation conversion table responsive to the characteristics of the original image to be processed. However, the image processing apparatus adapted to generate an apropriate gradation correction table responsive to the characteristics of the original image is complicated in structure.
Therefore, efforts have been made to develop a technique involving a simple structure for obtaining a gradation correction table which is responsive to the characteristics of each original. For example, a method for determining a gradation correction curve on the basis of a density histogram of an image has been proposed. This method is based on a technical concept of assigning a large number of gradation levels to a density range in which a number of pixels are concentrated in order to provide clear discrimination of fine differences in density to such a density range.
In this method, density histograms of images are first classified into a plurality of types by reading various standard pieces of printed matter. The types of the density histograms are represented by modeled density histogram patterns (standard histogram patterns) respectively, and a gradation correction curve is prepared for each standard histogram pattern in response to the characteristic of that pattern.
Then, a density histogram is obtained with respect to an original to be actually read. This density histogram is visually compared with the respective standard histogram patterns by an operator. The standard histogram pattern, which is most similar in configuration to the density histogram of the original to be read, is selected to perfom gradation correction on the basis of the gradation correction curve corresponding to the standard histogram patterns thus selected.
In this method, however, a number of standard histogram patterns must be obtained in advance in order to provide the most appropriate gradation correction curve for each original. As a result, the work required by the operator for determining the configuration and the number of the standard histogram patterns are substantial, and this increases costs. Further, memory capacity required for such jobs is also increased.
In another conventional technique, gradation correction curves themselves are modeled to prepare a plurality of types of standard gradation correction curves. When the standard gradation correction curves are modified so that density values designated "highlight" and "shadow points" are corrected to prescribed output values, respectively. Thereafter some density values are additionally selected as check points. With respect to the check points thus set, deviations between the standard gradation correction curves and desired correction characteristics are automatically calculated through an arithmetic unit. Then, a standard gradation correction curve having the minimum sum of deviations with respect to the respective check points are selected and employed for an actual gradation correction.
The second technique requires no processing of the operator's visual observation of the density distribution of the original. However, as in the aforementioned first method, this technique is also inconvenient since a number of the standard gradation correction curves must be prepared, and required memory capacity is also increased as the number of types is increased. Further, when the numbers of check points and standard gradation correction curves are increased in order to more appropriately select the standard gradation correction curve with respect to the original to be read, the operating time for arithmetic is increased, thereby delaying the processing.